This invention relates to transcutaneous pacemakers and defibrillators.
Emergency techniques for cardiac therapy are essential for successfully treating life threatening cardiac conditions. The most common of such conditions is ventricular fibrillation, in which the electrical pulse generators in the cardiac muscle fibrillate asynchronously, causing chaotic muscle contraction. The other common cardiac threat is loss of pacing, in which the pacing stimulus nerves of the cardiac muscle fail to initiate contraction of the muscle.
Ventricular fibrillation is treated with a high energy electrical pulse, called a defibrillation pulse, which is transcutaneously delivered to the heart to resynchronize the heart's pulse generators. Loss of pacing is treated using a transcutaneous pace maker to deliver pacing current pulses to the heart and thereby maintain cardiac contractions.
Frequently a patient in cardiac distress experiences both the conditions of fibrillation and loss of pacing. In order to most efficiently treat this situation, both defibrillation and external pacing equipment are typically combined in a single portable instrument for emergency personnel convenience. Such an instrument includes a pair of transcutaneous pacing electrodes and corresponding pacing circuitry, as well as a pair of defibrillation electrodes and corresponding defibrillation circuitry. In addition, the instrument may include a specialized multifunction electrode pair which can deliver both pacing and defibrillation pulses when used with appropriate connections to the pacing and defibrillation circuitry.
Typically, a hardware relay scheme isolates operation of the pacing circuitry from the defibrillation circuitry to thereby isolate the circuitry outputs. When separate pairs of pacing and defibrillation electrodes are used with the circuitry, the relay scheme decouples the two circuits. Conversely, when the multifunction electrode pair is used, the relay scheme couples the circuits to a common output so that the pacing stimuli and defibrillation pulses may be delivered to a single electrode pair. Such relays, being required to withstand the defibrillation pulse, are quite bulky and rather expensive, and must be isolated from any surrounding transformers that could trigger their activation.
Defibrillation electrodes are usually mounted one each on a hand-held paddle which includes a pressure-activated switch for initiating the defibrillation discharge of energy to a patient. Because this energy is high enough to be lethal if delivered at the wrong time or to the wrong location, the paddle switches are both connected in series with the discharge circuitry. If one switch is unintentionally activated alone, no discharge will result; only the simultaneous activation of both switches will initiate a discharge. However, if one switch is held closed (activated) as a result of a hardware malfunction, activation of the other switch would result in unintentional defibrillation discharge. Without the ability to monitor the open and closed state of the switches an operator has no way of verifying the integrity of the switches, and therefore the integrity of the discharge circuit. Moreover, the discharge control circuitry is unable to internally detect a switch malfunction and prevent a corresponding discharge malfunction.
The electrical current generated by the defibrillation discharge circuit for delivery to the patient is typically monitored to check the integrity of the circuit and to study the physiological effects of the discharge. This current may reach as much as 125 A, and the corresponding voltage of the discharge may reach as much as 5000 V. Conventionally, the current is monitored using a current transformer whose primary winding is connected in series with the discharge circuit. The secondary winding of the transformer is then connected to a sensing circuit for measuring the current level. The particular choice of transformer is based on the requirement that the component withstand the high current and voltage levels of the discharge circuit; typically such a transformer is disproportionately larger than the other circuit components and is rather expensive. More importantly, such a transformer generates substantial coupling flux between its primary and secondary windings which can severely impact the operation of neighboring circuits. The transformer must thus be physically isolated from circuits having sensitivity to the coupling flux.
It is desirable to frequently test the defibrillator discharge circuit for functionality, due to the critical nature of the emergency situations in which it is needed. Traditionally, such a test is accommodated by providing a characteristic load resistor, say 50 .OMEGA., into which the circuit may be energized to simulate discharge into a patient. The 50 .OMEGA. load resistor is typically contained either within the defibrillator equipment or within a separate testing apparatus, with a connection through the equipment housing for contact to the defibrillator electrodes.
Because the defibrillator discharge's high energy (about 360 j) is delivered in a short time (90% in about 3 msec), the peak power requirement of the load resistor is quite substantial, exceeding 100 KW. The resistor must be, well ventilated because the average power may exceed 30 watts, resulting in significant resistive heating, and typically, some type of customized heat sink is required in the location of the resistor. Additionally, a sizeable resistor is required to withstand the discharge's peak voltage of as much as 2500 V. This physical size may use up a substantial portion of the circuitry housing. The high voltage also requires the electrical connection from the defibrillator electrodes to the resistor to regularly withstand such voltage. Finally, the electrical connection on the equipment housing to the resistor must be isolated to prevent accidental contact by an operator; discharge of the circuit with only one electrode connected to the resistor while an operator touches the resistor would dump the high voltage across the resistor to the operator.